A University of Eastern Finland study has found that the fish stocking practice, a fisheries compensation method in hydropower operations, is failing to meet the latest legal and scientific requirements.

Stocking is a method of raising fish in a hatchery and releasing them into a river, lake, or the ocean to supplement existing populations, or to create a population where none exists.

Published in ‘Water International’, the study focuses on ecological flows relating to law and biology.

“As fish adapt to the farm environment, their genetic traits change and they no longer survive in the wild as well as they used to.”

The majority of Finnish rivers were used for hydropower operations after the two world wars. In order to compensate potential harm caused by the hydropower to fisheries, a Finnish legislation made it mandatory to build fishways.

Fishways were then rapidly replaced by fish farming as the compensation method. Fish stocking was among the first examples of ecological compensation measures set for the industries.

However, research found that fish stocking does not have the potential to permanently replace the natural reproductive cycles of migratory fish.

Before the industrial revolution, Finland had 25 rivers used by mating Atlantic salmon, with 72 that were home to anadromous brown trout. This was in addition to dozens of inland rivers with landlocked salmonids.

Currently, the rivers with natural reproduction of salmon have been reduced to four, and only a small population of wild migratory brown trout, migratory whitefish and grayling have survived.

University of Eastern Finland Fish Biology senior researcher Hannu Huuskonen said: “The problems of migratory fish in Finland are not limited to large coastal rivers.

“For instance, populations of salmonids and migratory whitefish living in our inland watersheds are in desperate need of new reproduction areas and migratory routes.”

The study concluded that the harm caused by hydropower operations cannot be compensated sustainably without the natural reproduction of fish. It suggests that in the future, the harm caused by hydropower operations should be compensated with the entire aquatic ecosystem in mind and not just fish.